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The tropics in a changing climate

This analysis explores the mechanisms behind changes in tropical precipitation in a warming climate, focusing on two primary components: the thermodynamic effect due to increased moisture and the dynamic effect of deepened convection. Using data from the IPCC and studies by Chou et al., the paper addresses how global warming and El Niño phenomena influence precipitation frequency and intensity. Key findings indicate an increase in heavy rainfall frequency while light precipitation diminishes, and overall precipitation intensity may experience notable changes due to the thermodynamic and dynamic interactions in the tropical atmosphere.

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The tropics in a changing climate

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  1. The tropics in a changing climate Chia Chou Research Center for Environmental Changes Academia Sinica October 19, 2010 NCU

  2. Mechanisms of mean tropical precipitation changes Chou et al. 2009

  3. Based on these mechanisms, to further examine changes in the tropics: — the direct moisture effect (thermodynamic component) — the effect of deepened convection — changes in precipitation intensity and frequency

  4. The direct moisture effect (changes in moisture)

  5. IPCC AR4

  6. IPCC AR4

  7. Column water vapor changes(IPCC AR4 ensemble)

  8. The Hadley circulation

  9. Precipitation changes (end of 21st century)

  10. widening of annual range

  11. horizontal moisture advection vertical moisture advection P : precipitation E : evaporation ω: vertical velocity q: Lq, moisture

  12. Hadley circulation Precipitation change in a warmer climate Lq    E, LW, H

  13. 1997-98 El Niño

  14. ( (850~200 ) MSU El Niño

  15. SST

  16. Spatial asymmetry ( for 8283 ,9192,9798) CMAP Annual cycle of basic state

  17. Vertically integrated moist static energy budget h : moist static energy Fnet : net flux into the atmosphere ω: vertical velocity q: Lq, moisture T: CpT, temperature

  19. El Niño

  20. Conclusion 1 • Asymmetry of mean tropical precipitation changes  widening of annual precipitation range • Mechanisms  Global warming: thermodynamic component dominates  ENSO: dynamic component dominates

  21. The effect of deepened convection

  22. Global water vapor budget (Held and Soden 2006):  thermodynamic dynamic P: precipitation M: mass flux; q: PBL water vapor

  23. 1-3% in P per 1ºC T (model simulations)   <0  slowing of tropical circulation dynamic component   7.5% in q per 1ºC T (Clausius-Clapeyron) thermodynamic component Held and Soden (2006); Vecchi and Soden (2007)

  24. In global average, P = E P ≈ LW+SW (assuming H is small) Vecchi and Soden (2007)

  25. increases at 1-3% per 1ºC T increases at 7.5% per 1ºC T ?  NO 

  26. Vertically integrated water vapor budget convergence of moisture flux P: precipitation; E: evaporation q: water vapor (moisture); v: horizontal velocity ω: vertical velocity; ‹ ›: vertical integration

  27. Vertically integrated water vapor budget

  28. vertical advection horizontal advection  thermodynamic dynamic 

  30.  :a weakening of tropical circulation

  31.   >0 or <0 No constraint  7.5% in q per 1ºC T 7.5% in q per 1ºC T   <0  1-3% in P per 1ºC T (controlled by energy budget)

  32. Effect of convection depth

  33.  deepening of convection:~ 2.5-3.4%

  34. 155 hPa 150 hPa 145 hPa 141 hPa 137 hPa Convection top: 155 hPa ~ 137 hPa (-1.2% ~ 3.3%)

  35. Convection top: 155 hPa ~ 137 hPa (-1.2% ~ 3.3%) Chou and Chen 2010

  36. Deeper convection more E less E Reduced upward motion; Less convergence of moisture flux more evaporation

  37. Conclusion 2 • Effect of convection depth: the deeper (shallower) convection, the weaker (stronger) the circulation strength of tropical circulation: atmospheric stability; upper troposphere

  38. Changes in precipitation frequency and intensity

  39. Precipitation Frequency • Scatterplot of model-simulated percentage change (%) for globally averages

  40. Precipitation frequency

  41. Precipitation Intensity • Scatterplot of model-simulated percentage change (%) for globally averages

  42. Precipitation Intensity

  43. Conclusion 3 • Frequency is enhanced for median and heavy precipitation, while reduced for light precipitation • Intensity is enhanced for heavy precipitation, but inconsistent for median and light precipitation

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